Network Applications: UDP and TCP Socket Programming Y. Richard Yang 9/17/2013

2 Outline  Recap r Network application programming: intro to sockets m UDP m TCP

3 Recap: DNS r Function m map between (domain name, service) to value, e.g., ( Addr) -> (cs.yale.edu, ) -> netra.cs.yale.edu

4 Recap: DNS r Key design features of DNS m Hierarchical domain name space allowing delegation m Recursive or iterative queries

5 Recap: DatagramSocket(Java) r DatagramSocket() constructs a datagram socket and binds it to any available port on the local host r DatagramSocket(int lport) constructs a datagram socket and binds it to the specified port on the local host machine. r DatagramSocket(int lport, InetAddress laddr) creates a datagram socket and binds to the specified local port and laddress. r DatagramSocket(SocketAddress bindaddr) creates a datagram socket and binds to the specified local socket address. r DatagramPacket(byte[] buf, int length) constructs a DatagramPacket for receiving packets of length length. r DatagramPacket(byte[] buf, int length, InetAddress address, int port) constructs a datagram packet for sending packets of length length to the specified port number on the specified host. r receive(DatagramPacket p) receives a datagram packet from this socket. r send(DatagramPacket p) sends a datagram packet from this socket. r close() closes this datagram socket.

Connectionless UDP: Big Picture (Java version) close clientSocket Server (running on serv ) read reply from clientSocket create socket, clientSocket = DatagramSocket() Client Create datagram using ( serv, x) as (dest addr. port), send request using clientSocket create socket, port= x, for incoming request: serverSocket = DatagramSocket( x ) read request from serverSocket write reply to serverSocket generate reply, create datagram using client host address, port number r Create socket with port number: DatagramSocket sSock = new DatagramSocket(9876); r If no port number is specified, the OS will pick one

7 Example: UDPServer.java r A simple UDP server which changes any received sentence to upper case.

8 Java Server (UDP): Create Socket import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); Create datagram socket bind at port 9876 Check socket state: %netstat –p udp –n

9 System State after the Call server UDP socket space address: {*:9876} snd/recv buf: address: { :53} snd/recv buf: “*” indicates that the socket binds to all IP addresses of the machine: % ifconfig -a local address why shown as “*”? local port

10 Binding to IP Addresses server UDP socket space Public address: Local address: address: { :53} snd/recv buf: InetAddress sIP1 = InetAddress.getByName(“localhost”); DatagramSocket ssock1 = new DatagramSocket(9876, sIP1); InetAddress sIP2 = InetAddress.getByName(“ ”); DatagramSocket ssock2 = new DatagramSocket(9876,sIP2); DatagramSocket serverSocket = new DatagramSocket(6789); address: { :9876} snd/recv buf: address: { :9876} snd/recv buf: address: {*:6789} snd/recv buf:

11 UDP Demultiplexing server UDP socket space Public address: Local address: address: { :53} snd/recv buf: UDP demutiplexing is based on matching (dst address, dst port) address: { :9876} snd/recv buf: address: { :9876} snd/recv buf: P1 client on server SP: x DP: 9876 S-IP: A D-IP: P2 client IP: B SP: y DP: 9876 S-IP: B D-IP:

12 UDP Demultiplexing server UDP socket space Public address: Local address: address: { :53} snd/recv buf: UDP demutiplexing is based on matching (dst address, dst port) P1 Client on server SP: x DP: 9876 S-IP: A D-IP: P3 client IP: C SP: y DP: 6789 S-IP: C D-IP: address: { :9876} snd/recv buf: address: { :9876} snd/recv buf: address: {*:6789} snd/recv buf:

Per Socket State r Each socket has a set of states: m local address m receive buffer size m send buffer size m timeout See DatagramSocket API to display socket state. Example: socket state after clients sent msgs to the server 13

14 Java Server (UDP): Receiving import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = null; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket); Create space for received datagram Receive datagram

15 Java Server (UDP): Processing import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { … // process data String sentence = new String(receivePacket.getData(), 0, receivePacket.getLength()); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); getData() returns a pointer to an underlying buffer array; for efficiency, don’t assume receive() will reset the rest of the array getLength() returns how much data is valid.

16 Java Server (UDP): Response r Java DatagramPacket:  getAddress()/getPort () returns the source address/port

17 Java server (UDP): Reply InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } Get IP addr port #, of sender Write out datagram to socket End of while loop, loop back and wait for another datagram Create datagram to send to client

18 Example: UDPClient.java r A simple UDP client which reads input from keyboard, sends the input to server, and reads the reply back from the server.

19 Example: Java client (UDP) import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); String sentence = inFromUser.readLine(); byte[] sendData = sentence.getBytes(); DatagramSocket clientSocket = new DatagramSocket(); InetAddress sIPAddress = InetAddress.getByName(“servname"); Create input stream Create client socket Translate hostname to IP address using DNS

20 Example: Java client (UDP), cont. DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, sIPAddress, 9876); clientSocket.send(sendPacket); byte[] receiveData = new byte[1024]; DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } Create datagram with data-to-send, length, IP addr, port Send datagram to server Read datagram from server

Discussion on Example Code r A simple upper-case echo service is among the simplest network service. r Are there any problems with the program? 21

Data Encoding/Decoding r Pay attention to encoding/decoding of data: transport layer handles only a sequence of bytes 22 byte array query encoding decoding client server if not careful, query sent != query received (how?) result

Example: Endianness of Numbers r int var = 0x0A0B0C0D 23 Intel x86 ARM, Power PC, Motorola 68k, IA-64

Example: String and Chars 24 String byte array String.getBytes() client server Will we always get back the same string? String(rcvPkt, 0, rcvPkt.getLength()); Depends on default local platform char set (why?) : java.nio.charset.Charset.defaultCharset() (UTF-16) String (UTF-16)

Example: Charset Troubles r Try m java EncodingDecoding US-ASCII UTF-8 m java EncodingDecoding ISO UTF-8 25

Encoding/Decoding as a Common Source of Errors r Please read chapter 4 of Java Network Programming for more details r Common mistake even in many (textbook) examples: m Protocol/UseDatagramSockettosendoutandrece iveDatagramPacket.htm 26

Discussion: UDP/DNS Server Pseudocode 27 r Modify the example UDP server code to implement a DNS server. r flags: m query or reply m recursion desired m recursion available m reply is authoritative

28 UDP/DNS Implementation r Standard UDP demultiplexing (find out return address by src.addr/src.port of UDP packet) does not always work r DNS solution: identification: remember the mapping

Outline r Recap r Network application programming m UDP  TCP 29

TCP Socket Design: Starting w/ UDP server Socket socket space address: {*:9876} snd/recv buf: local address local port Issue: TCP is designed to provide a pipe abstraction: server reads an ordered sequence of bytes from each connected client Issue 2: How is the server notified that a new client is connected? P1 client1 P2 client2 sock.nextByte(client1)? newClient = sock.getNewClient()?

BSD TCP Socket API Design server TCP socket space Q: How to decide where to put a new packet? address: {*:9876} snd/recv buf: address: {*:9876; client 1} snd/recv buf: address: {*:9876; client 2} snd/recv buf: socket for new connected clients P1 client1 P2 client2 A: Packet demutiplexing is based on four tuples: (dst addr, dst port, src addr, src port)

TCP Connection-Oriented Demux r TCP socket identified by 4-tuple: m source IP address m source port number m dest IP address m dest port number r recv host uses all four values to direct segment to appropriate socket m server can easily support many simultaneous TCP sockets: different connections/sessions are automatically separated into different sockets

33 Connection-Oriented Demux Client IP:B P1 client IP: A P1P2P4 server IP: S SP: x DP: 25 SP: y DP: 25 P5P6P3 D-IP: S S-IP: A D-IP: S S-IP: B SP: x DP: 25 D-IP: S S-IP: B

34 TCP Socket Big Picture

Summary: Socket Programming with TCP Client must contact server r server process must first be running r server must have created socket (door) that welcomes client’s contact Client contacts server by: r creating client-local TCP socket r specifying IP address, port number of server process r When client creates socket: client TCP establishes connection to server TCP r When contacted by client, server TCP creates new socket for server process to communicate with client TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server application viewpoint

36 Connection-oriented TCP: Big Picture (C version) welcomeSocket=socket(): create socket bind(welcomeSocket, …): specify socket address/port server client TCP connection setup listen(welcomeSocket, …): specify that socket welcomeSocket is a listening socket connectionSocket=accept(welcomeSocket, …): get a connected connection from the queue for socket welcomeSocket; create a new socket identified by connectionSocket read()/write(): do IO on socket connectionSocket close(connectionSocket): done clientSocket=socket(): create socket bind(clientSocket): specify socket address connect(clientSocket, serverAddr, serverPort): initialize TCP handshake to server; return until TCP handshake is done read()/write(): do IO on clientSocket close(clientSocket): done optional

Client/server socket interaction: TCP wait for incoming connection request connectionSocket = welcomeSocket.accept() create socket, port= x, for incoming request: welcomeSocket = ServerSocket(x) create socket, connect to hostid, port= x clientSocket = Socket() close connectionSocket read reply from clientSocket close clientSocket Server (running on hostid ) Client send request using clientSocket read request from connectionSocket write reply to connectionSocket TCP connection setup

Server Flow connSocket = accept() Create ServerSocket(6789) read request from connSocket Serve the request close connSocket -Welcome socket: the waiting room -connSocket: the operation room

39 ServerSocket r ServerSocket() m creates an unbound server socket. r ServerSocket(int port) m creates a server socket, bound to the specified port. r ServerSocket(int port, int backlog) m creates a server socket and binds it to the specified local port number, with the specified backlog. r ServerSocket(int port, int backlog, InetAddress bindAddr) m create a server with the specified port, listen backlog, and local IP address to bind to. r bind(SocketAddress endpoint) m binds the ServerSocket to a specific address (IP address and port number). r bind(SocketAddress endpoint, int backlog) m binds the ServerSocket to a specific address (IP address and port number). r Socket accept() m listens for a connection to be made to this socket and accepts it. r close() closes this socket.

40 (Client)Socket r Socket(InetAddress address, int port) creates a stream socket and connects it to the specified port number at the specified IP address. r Socket(InetAddress address, int port, InetAddress localAddr, int localPort) creates a socket and connects it to the specified remote address on the specified remote port. r Socket(String host, int port) creates a stream socket and connects it to the specified port number on the named host. r bind(SocketAddress bindpoint) binds the socket to a local address. r connect(SocketAddress endpoint) connects this socket to the server. r connect(SocketAddress endpoint, int timeout) connects this socket to the server with a specified timeout value. r InputStreamget InputStream() returns an input stream for this socket. r OutputStreamgetOutputStream() returns an output stream for this socket. r close() closes this socket.

OutputStream r public abstract class OutputStream m public abstract void write(int b) throws IOException m public void write(byte[] data) throws IOException m public void write(byte[] data, int offset, int length) throws IOException m public void flush( ) throws IOException m public void close( ) throws IOException 41

InputStream r public abstract class InputStream m public abstract int read( ) throws IOException m public int read(byte[] input) throws IOException m public int read(byte[] input, int offset, int length) throws IOException m public long skip(long n) throws IOException m public int available( ) throws IOException m public void close( ) throws IOException 42

43 TCP Example Example client-server app: 1) client reads line from standard input ( inFromUser stream), sends to server via socket ( outToServer stream) 2) server reads line from socket 3) server converts line to uppercase, sends back to client 4) client reads, prints modified line from socket ( inFromServer stream)

Example: Java client (TCP) import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); sentence = inFromUser.readLine(); Socket clientSocket = new Socket(“server.name", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream()); Create input stream Create client socket, connect to server Create output stream attached to socket

Example: Java client (TCP), cont. outToServer.writeBytes(sentence + '\n'); BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); modifiedSentence = inFromServer.readLine(); System.out.println ("FROM SERVER: " + modifiedSentence ); clientSocket.close(); } Read line from server Create input stream attached to socket Send line to server

Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Create welcoming socket at port 6789

Under the Hood: TCP Multiplexing server client TCP socket space state: listening address: {*:6789, *:*} completed connection queue: sendbuf: recvbuf: TCP socket space state: listening address: {*:25, *:*} completed connection queue: sendbuf: recvbuf: state: listening address: {*:25, *:*} completed connection queue: sendbuf: recvbuf: state: starting address: { :1500, *:*} sendbuf: recvbuf: local addr local port remote addr remote port %netstat –p tcp –n -a

Example: Client Initiates Connection server client TCP socket space state: listening address: {*:6789, *.*} completed connection queue: sendbuf: recvbuf: TCP socket space state: listening address: {*.25, *.*} completed connection queue: sendbuf: recvbuf: state: listening address: {*.25, *.*} completed connection queue: sendbuf: recvbuf: state: connecting address: { :1500, :6789} sendbuf: recvbuf:

Example: TCP Handshake Done server client TCP socket space state: listening address: {*:6789, *:*} completed connection queue: { , } sendbuf: recvbuf: TCP socket space state: listening address: {*:25, *:*} completed connection queue: sendbuf: recvbuf: state: listening address: {*:25, *:*} completed connection queue: sendbuf: recvbuf: state: connected address: { :1500, :6789} sendbuf: recvbuf:

Example: Java server (TCP) import java.io.*; import java.net.*; class TCPServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept(); BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); Wait, on welcoming socket for contact by client

Example: Server accept() server client TCP socket space state: listening address: {*.6789, *:*} completed connection queue: sendbuf: recvbuf: TCP socket space state: listening address: {*.25, *:*} completed connection queue: sendbuf: recvbuf: state: listening address: {*.25, *:*} completed connection queue: sendbuf: recvbuf: state: connected address: { , :6789} sendbuf: recvbuf: state: established address: { :6789, } sendbuf: recvbuf: Packet sent to the socket with the best match! Packet demutiplexing is based on (dst addr, dst port, src addr, src port)

Example: Java server (TCP): Processing BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; DataOutputStream outToClient = new DataOutputStream (connectionSocket.getOutputStream()); outToClient.writeBytes(capitalizedSentence); } Read in line from socket Create input stream, attached to socket

Example: Java server (TCP): Output BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream())); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; DataOutputStream outToClient = new DataOutputStream (connectionSocket.getOutputStream()); outToClient.writeBytes(capitalizedSentence); } Create output stream, attached to socket Write out line to socket End of while loop, loop back and wait for another client connection

Analysis r Assume that client requests arrive at a rate of lambda/second r Assume that each request takes 1/mu seconds r Some basic questions m How long is the queue at the welcome socket? m What is the response time of a request? 54 Welcome Socket Queue

Analysis r Is there any interop issue in the sample program? m DataOutputStream writeBytes(String) truncates – tputStream.html#writeBytes(java.lang.String) 55